Microstructural Aspects of Bainitic and Bainite-like Ferritic Structures of Continuously Cooled Low Carbon (<0.1%) HSLA Steels

Abstract

Microstructures of modern thermo-mechanical control processed very low carbon HSLA steels are investigated by focusing into the intermediate stage transformed (Zw-)structures which have been hitherto regarded as being difficult to identify for classification. Main influencing factors for the formation of Zw-ferritic matrix-phases are analyzed and discussed. These Zw-matrices are classified into αq, αw and αBo respectively as an almost composition-invariant transformation product in the case of ~0.01% carbon content. Further consideration on the relationship between characteristic features and transformation modes seems helpful for interpreting the complicated state of such classified microstructures. Nucleation and growth modes of each phase beside purely diffusional and martensitic phases are discussed and summarized in both cases of as transformed from recrystallized and from unrecrystallized austenite. A little higher carbon over 0.02 up to 0.08% results in a small portion of dispersed C-enriched microconstituents accompanying the Zw-matrices. © 1991, The Japan Institute of Metals. All rights reserved.